I agree that your explanation of the rift is the best guess as to what the show runners intended, but I think we got really uneven exposition. In the beginning, Shae got a ton of time and the actress played her loyalty to Tyrion as based in love. To me this comes across clearly in her acting choices, and if you read interviews with the actress, it's how she sees the relationship too. It's only because of this depth early on that I particularly feel the characters and the dissolution of the relationship deserved more time -- if Shae had been book Shae, I would care less about exactly what had gone on in her head since we saw her head for the boat. But in particular, I also want to know what Tyrion thinks about their relationship, too. The whole murder was played, to me, very perfunctorily, when in fact there are many more possible interactions that seem more likely to me (consistent with the characters) than what happened. Not at all a book purist, but the combination of the different characterization combined with sudden course correction to a "set piece" from the book just felt emotionally wrong to me. I needed more exposition for two reasons: 1) explicit confirmation that the show remembers the early portrayal of their relationship, vs. just retconning to get to the book outcome and 2) book aside, more emotional satisfaction from the dialogue and/or acting (I think PD is better at some emotions than others - he just doesn't do temporarily unhinged as well). I was invested in their relationship, and simply don't find it satisfying to fill in the details myself in this case.
Prior to this episode, I wouldn't have minded if Shae had taken Tysha's place in Tyrion's emotional baggage going forward, but based on how everything went down in the end, it's probably going to feel pretty unsatisfying if they go this direction unless we get more explanation than we currently have.